The magnetic bearing technology includes fields of application of machine and apparatus construction with extremely high requirements for the rotary speed range, the working life, the cleanliness and the sealed nature of the drive system—i.e. essentially fields of application which cannot be realized or can only be realized with difficulty using conventional bearing techniques. Various embodiments, such as for example high speed milling and grinding spindles, turbo compressors, vacuum pumps or pumps for high purity chemical or medical products, are already equipped with magnetic bearings.
The machine cross-sections shown in the following figures are simply by way of example and partly greatly simplified and serve exclusively for the more precise explanation of the principle of operation.
A conventional magnetically journalled machine (FIG. 1) requires, in addition to a machine unit (1), two radial magnetic bearings (2), (3), an axial magnetic bearing (4), two mechanical touch down bearings (5), (6), as well as a total of ten power converter stages (7), (8), (9), (10) for the control of the motor phases and magnetic bearing phases.
In the literature there are proposals (FIG. 2) for the integration of machines and radial magnetic bearings into one magnetic stator unit. In one stator there are two separate winding systems (11), (12) for torque winding and levitation force winding which are inserted into slots in multiple layers. The relationship p1=p2±2 basically applies in bearingless motors for a largely decoupled levitation force formation and torque formation between the winding pole numbers, with p1 or p2 also representing the pole number of the rotor. Both winding systems of the motor in FIG. 2 are three-phase. The coils are chord-wound and distributed over several slots, whereby an approximately sinusoidal flux linkage is achieved. The two windings are composed as follows:    four-pole machine winding (11) (outer): phase 1 (13), phase 2 (14), phase 3 (15)    two-pole levitation winding (12) (inner): phase 1 (16), phase 2 (17), phase 3 (18).
In order to achieve a cost-favorable overall system the possibility exists of reducing the number of winding systems and thus to simplify the control electronics in addition to the mechanical layout.
As an example of a motor with a reduced number of windings, a motor with common torque winding systems and levitation force winding systems consisting of four concentrated coils should be explained which will be termed a bearingless single-phase motor in the following.
In FIG. 3 the rotor and stator of a four-pole motor is shown in an external rotor embodiment. In this arrangement the rotor (35) is preferably constructed in ring-shaped or bell-shaped design. With the aid of the four concentrated coils (31, 32, 33, 34) generation of a two-pole and four-pole circulation distribution is possible, so that a levitation force in the x and y directions and torque can be produced independently of one another. The determination of the individual phase currents takes place paying attention to the desired value setting for the rotor position and speed of rotation, rotor angle or torque after evaluation of the sensor signals for rotor position (x, y) and rotor angle of rotation (φ).
In the preceding section in connection with the prior art the bearingless single-phase motor and the multiphase rotary field motors were described.
Both embodiments have considerable technical and economical disadvantages:
The bearingless single-phase motor (FIG. 3) is only suited for applications with low requirements with respect to the starting torque. These include, for example, drives for pumps, blowers, fans or ventilators. In the simplest constructional form the bearingless single-phase motor requires only four individual coils. The starting weakness of the single-phase drive is brought about by the design. Whereas rotary field windings are used for the building up of the radial levitation forces, the motor part only has one single-phase alternating field winding. There are thus critical angular positions of the rotor in which the starting torque is zero independently of the selected current amplitude. Provision must therefore be made design-wise for the rotor to come to rest only in positions which differ from the critical angular positions. The moment of inertia of the drive thus enables the critical points to be overcome in particular in the starting phase but also in the steady-state operation. For many other drive tasks the starting torque of this type of motor is too small.
The bearingless multiphase motor (FIG. 2) corresponding to the prior art does not have the disadvantage of low torque angular positions. In contrast to the single-phase variant it not only has its own rotary field winding in the bearing part but also in the motor part. However the high coil numbers associated with the two rotary field windings are disadvantageous here. Typical coil numbers of such bearingless motors range as a rule between 36(distributed three-phase windings) and 12 (simple two-phase windings).
Bearingless motors, with small numbers of coils and without the restrictions of the field of application which result from the single-phase technology, would be desirable from the preceding considerations and technically and economically extremely interesting for the drive market.